Collision Responses

I've been trying to port my game from the old EA edition up to the Beta. I've been doing okay with a few bumps here and there until I came to Collision Responses. They seem nice but I haven't been able to find any real documentation on them. Until now I've simply set tags on objects and used the onCollision call back. When two object collide I just check their tags and call the appropriate functions. Now everything by default uses the clamp response. That's okay, but what if I want an object to clamp on some things and bounce on others? And what if I don't want anything at all to happen except for the call back? I guess what I'm asking is, can I get the call back to fire without an automated response?

Here's my situation. I have two objects: a player and a tree. The camera is directly above. Now when the player drives under the tree I want the two to collide so that I can make the tree fade out. But I don't want the player's movement hindered in any way. I've tried every combination of send and recieve imaginable with no success. On the other hand the player might run into a block, in which case the clamp response works fine. So how do I get this to work? Is there some documentation on the new collision responses somewhere that I missed?

-Peter